Before I explain, let me first say that I HAVE been
infected by a virus, which is why I got Norton,
but I'm now clean... point being, I'm with you...
I don't want to turn off anything.

But, my confusion revolves about "who" is doing
the virus scan  when I load an Excel spreadsheet:

Norton or Microsoft?

Here's what threw me... your reply got me thinking
about Norton, so as an experiment, I disabled
AutoProtect as well as Script Blocking. I then
rebooted. I then launched Excel from my desktop
(i.e., a totally new, blank workbook). The phrase
"running virus scan..." still appeared at the bottom.
What gives? It's totally blank !!